Bipolar transistor, semiconductor device, and bipolar transistor manufacturing method

ABSTRACT

Disconnection of a base line is suppressed even when a short-side direction of a collector layer is parallel to crystal orientation [011]. A bipolar transistor includes: a collector layer that has a long-side direction and a short-side direction in a plan view, in which the short-side direction is parallel to crystal orientation [011], a cross-section perpendicular to the short-side direction has an inverted mesa shape, and a cross-section perpendicular to the long-side direction has a forward mesa shape; a base layer that is formed on the collector layer; a base electrode that is formed on the base layer; and a base line that is connected to the base electrode and that is drawn out from an end in the short-side direction of the collector layer to the outside of the collector layer in a plan view.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. patent application Ser. No. 15/073,802, filedMar. 18, 2016, which is a division of U.S. patent application Ser. No.14/328,737 filed Jul. 1, 2014, the entire disclosures of both of whichare incorporated by reference herein.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a bipolar transistor, a semiconductordevice, and a bipolar transistor manufacturing method.

Background Art

In the related art, development of a bipolar transistor having acollector layer, a base layer, and an emitter layer on a substrate hasbeen attempted.

Patent Document 1 discloses a bipolar transistor in which a base lineconnected to a base electrode coming into contact with a base layer isdrawn out from an end in a long-side direction of a rectangularcollector layer in a plan view.

CITATION LIST Patent Document

[Patent Document 1] JP2004-327904 A

SUMMARY OF THE INVENTION

However, in the bipolar transistor described in Patent Document 1, sincethe collector layer is wet-etched, etching anisotropy may occurdepending on crystal orientations of the collector layer. As a result,in the collector layer, a cross-section perpendicular to crystalorientation [011] has an inverted mesa shape and a cross-sectionextending along crystal orientation [011] has a forward mesa shape.

Here, the base line is drawn out from an end in the long-side directionof the collector layer. Accordingly, when the short-side direction ofthe collector layer extends along crystal orientation [011], the baseline is drawn out from an end of the collector layer having an invertedmesa shape to the outside of the collector layer and thus may bedisconnected due to step difference of the mesa.

An object of the present invention is to suppress disconnection of abase line even when a short-side direction of a collector layer extendsalong crystal orientation [011].

According to an aspect of the present invention, there is provided abipolar transistor including: a collector layer that has a long-sidedirection and a short-side direction in a plan view, in which theshort-side direction is parallel to crystal orientation [011], across-section perpendicular to the short-side direction has an invertedmesa shape, and a cross-section perpendicular to the long-side directionhas a forward mesa shape; a base layer that is formed on the collectorlayer; a base electrode that is formed on the base layer; and a baseline that is connected to the base electrode and that is drawn out froman end in the short-side direction of the collector layer to the outsideof the collector layer in a plan view.

According to the present invention, it is possible to suppressdisconnection of a base line even when a short-side direction of acollector layer extends along crystal orientation [011].

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an HBT as an example of a bipolar transistoraccording to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along line A-A of FIG. 1.

FIG. 3 is a cross-sectional view taken along line B-B of FIG. 1.

FIG. 4 is a cross-sectional view taken along line C-C of FIG. 1.

FIG. 5 is a cross-sectional view taken along line D-D of FIG. 1.

FIG. 6 is a plan view of an HBT as an example of a bipolar transistoraccording to a second embodiment of the present invention.

FIG. 7 is a plan view of a semiconductor device according to a thirdembodiment of the present invention.

FIG. 8 is a plan view of a semiconductor device according to a fourthembodiment of the present invention.

FIG. 9 is a plan view of a semiconductor device according to a fifthembodiment of the present invention.

FIG. 10 is a cross-sectional view of a BiFET employing a base linedrawing method according to the first embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. The embodiments to be describedbelow are only examples and are not intended to exclude application ofvarious modifications or techniques not described below. That is, thepresent invention may be modified in various forms (for example,combinations of the embodiments) without departing from the gist of thepresent invention. In the following description with reference to theaccompanying drawings, identical or similar elements are referenced byidentical or similar reference signs. The drawings are schematic and arenot necessarily equal to actual dimensions, ratios, and the like. Partshaving different dimensional relationships or ratios may be present inthe drawings.

First Embodiment

A bipolar transistor according to a first embodiment of the presentinvention mainly includes a collector layer, a base layer, and anemitter layer on a substrate. In the first embodiment of the presentinvention, a hetero-junction bipolar transistor (hereinafter, referredto as “HBT”) in which at least one of a pair of the collector layer andthe base layer and a pair of the base layer and the emitter layer has ahetero-junction connection is described as an example of the bipolartransistor.

Structure

First, the structure of the HBT according to the first embodiment willbe described below. FIG. 1 is a plan view of an HBT 10 as an example ofthe bipolar transistor according to the first embodiment of the presentinvention. FIG. 2 is a cross-sectional view taken along line A-A ofFIG. 1. FIG. 3 is a cross-sectional view taken along line B-B of FIG. 1.FIG. 4 is a cross-sectional view taken along line C-C of FIG. 1. FIG. 5is a cross-sectional view taken along line D-D of FIG. 1.

The HBT 10 according to this embodiment is formed on a substrate, forexample, a panel-like compound semiconductor substrate 12.

The material of the compound semiconductor substrate 12 is notparticularly limited and an example thereof is a material having acrystal structure. Examples of the material having a crystal structureinclude GaAs, Si, InP, SiC, and GaN. Among these materials, it ispreferable that the substrate contains GaAs or Si, which is lower incost than InP or the like and easily increases in diameter, as a majorcomponent. Here, the “major component” means that the ratio of amaterial as a major component to a substrate or a layer as a whole is 80mass % or more. In this embodiment, the compound semiconductor substrate12 is formed of, for example, GaAs. In FIG. 1 or the like, crystalorientation [011], crystal orientation [010], and crystal orientation[01-1] are illustrated as some crystal orientations of GaAs.

The HBT 10 includes a sub collector layer 14, a collector layer 16, acollector electrode 18, a base layer 20, a base electrode 22, an emitterlayer 24, and an emitter electrode 26. The sub collector layer 14 isformed on a partial surface of the compound semiconductor substrate 12.The material of the sub collector layer 14 is not particularly limited,and an example thereof is a material having a crystal structure. A zincblende type crystal structure can be preferably used as the crystalstructure. In this embodiment, the sub collector layer 14 is formed of,for example, the same material as the compound semiconductor substrate12, which contains n-type GaAs (Si concentration of 5×10¹⁸ cm⁻³) as amajor component. The crystal orientation of GaAs of the sub collectorlayer 14 is parallel to, for example, the crystal orientation of GaAs ofthe compound semiconductor substrate 12.

The shape of the sub collector layer 14 in a plan view is, for example,a protrusion shape (see FIG. 1). The protruding direction of the subcollector layer 14 is substantially parallel to crystal orientation[01_-1] of the sub collector layer 14 (hereinafter, “substantiallyparallel to” is referred to as “extend along”). In the sub collectorlayer 14, a cross-section perpendicular to crystal orientation [011]thereof has an inverted mesa shape (see FIGS. 2 and 3) and across-section extending along crystal orientation [011] thereof has aforward mesa shape (see FIGS. 4 and 5). In other words, both sidesurfaces in the protruding direction of the sub collector layer 14 areinverted mesa surfaces, and both side surfaces in the width directionperpendicular to the protruding direction are forward mesa surfaces. Thethickness of the sub collector layer 14 is, for example, about 0.5 μm.

The “forward mesa shape” means a trapezoidal shape having a narrow top(the sub collector layer 14 side relative to the compound semiconductorsubstrate 12), and the “inverted mesa shape” means a trapezoidal shapehaving a broad top. The “forward mesa surface” means an inclined surfacewhich forms an obtuse angle with a top flat surface, and the “invertedmesa surface” means an inclined surface which forms an acute angle withthe top flat surface. The “substantially parallel” means that an angleabout a target parallel direction (for example, crystal orientation[01-1]) is 0 degrees±20 degrees. In this embodiment, more specifically,the protruding direction of the sub collector layer 14 is parallel tocrystal orientation [01-1] of the sub collector layer 14 (that is, theangle is 0 degrees). The sub collector layer 14 has low ohm resistanceand thus serves as a collector along with the collector layer 16.

The collector layer 16 is formed at the center in the width direction ofthe sub collector layer 14 on the sub collector layer 14 (see FIGS. 1and 2). The material of the collector layer 16 is not particularlylimited, and an example thereof is a material having a crystalstructure. A zinc blende type crystal structure can be preferably usedas the crystal structure. In this embodiment, the collector layer 16 isformed of, for example, the same material as the sub collector layer 14,which contains GaAs as a major component. The crystal orientation ofGaAs of the collector layer 16 is parallel to, for example, the crystalorientation of GaAs of the compound semiconductor substrate 12.

Since GaAs as the major component of the collector layer 16 has betterthermal conductivity than ordered InGaP which has been used in therelated art, a heat dissipation property to the collector layer 16 sideis improved and transistor characteristics in a high-temperatureoperation or a high-power operation are improved.

The collector layer 16 containing GaAs as a whole may be formed ofn-type semiconductor or p-type semiconductor. When the collector layer16 is formed of n-type semiconductor, the HBT 10 has an npn junction.When the collector layer 16 is formed of p-type semiconductor, the HBT10 has a pnp junction. Here, since the hole mobility of GaAs is muchlower than the electron mobility (the electron mobility is about 0.85m²/(V⋅s) and the hole mobility is about 0.04 m²/(V⋅s)), n-typesemiconductor can be preferably used from the viewpoint that thefrequency characteristics are more excellent than those of the pnpjunction. Hereinafter, it is assumed in this embodiment that thecollector layer 16 is formed of n-type semiconductor. In order to setthe collector layer 16 to the n type, the collector layer 16 is dopedwith dopants such as Si, S, Se, Te, and Sn. In order to set thecollector layer 16 to the p type, the collector layer 16 is doped withdopants such as C, Mg, Be, Zn, and Cd.

The shape of the collector layer 16 in a plan view is, for example, arectangular shape long in one direction (see FIG. 1). The long-sidedirection of the rectangular shape (collector layer 16) extends alongcrystal orientation [01-1] of the collector layer 16. The short-sidedirection of the collector layer 16 extends along crystal orientation[011] of the collector layer 16. The shape of the collector layer 16when viewed from crystal orientation [011] is an inverted mesa shape(see FIG. 2) and the shape thereof when viewed from crystal orientation[01-1] perpendicular to crystal orientation [011] is a forward mesashape (see FIG. 5). In other words, the cross-section perpendicular tothe short-side direction of the collector layer 16 has an inverted mesashape and the cross-section perpendicular to the long-side direction ofthe collector layer 16 is a forward mesa shape. In other words, bothside surfaces in the long-side direction of the collector layer 16 areinverted mesa surfaces and both side surfaces in the short-sidedirection of the collector layer 16 are forward mesa surfaces. Thethickness of the collector layer 16 is, for example, equal to or greaterthan 0.5 μm and equal to or less than 1.5 μm.

The collector electrode 18 (a pair) is formed on both sides in the widthdirection of the sub collector layer 14 with the collector layer 16interposed therebetween on the sub collector layer 14 (see FIG. 1).

The shape of each collector electrode 18 in a plan view is, for example,rectangular. The long-side direction of the rectangular shape (collectorelectrode 18) extends along crystal orientation [01-1] of the collectorlayer 16. The material of the collector electrodes 18 is notparticularly limited and examples thereof include Ti/Pt, WSi, Pt/Ti/Au,and AuGe/Ni/Au. Preferably, AuGe/Ni/Au can be used as the material ofthe collector electrodes 18. In a plan view, the pair of collectorelectrodes 18 retreats to crystal orientation [01-1] of the collectorlayer 16 with respect to the collector layer 16. In other words, thecollector layer 16 protrudes in the protruding direction of the subcollector layer 14 with respect to the pair of collector electrodes 18.

The collector electrodes 18 are connected to the same collector line 18Bvia respective contact holes 18A. The collector line 18B is connected toa collective collector line 19B of an upper layer via a contact hole 19Ain an upper layer thereon.

The contact holes 18A are formed of, for example, the same material asthe collector line 18B. The lateral outer circumference of each contacthole 18A is surrounded with, for example, a protective film as aninsulator not illustrated.

The shape of the collector line 18B in a plan view is, for example, arecess shape (see FIG. 1). The recessed portion of the recess shape isopened to the protruding portion of the sub collector layer 14. Therecessed portion surrounds a part of the base layer 20, the emitterlayer 24, or the like located at the center in the width direction ofthe sub collector layer 14.

The base layer 20 is formed on the collector layer 16 (see FIGS. 1 and2). The material of the base layer 20 is not particularly limited, andan example thereof is a material having a crystal structure. A zincblende type crystal structure can be preferably used as the crystalstructure. In this embodiment, the base layer 20 is formed of, forexample, the same material as the sub collector layer 14 and thecollector layer 16, which contains GaAs as a major component. Thecrystal orientation of GaAs of the base layer 20 is parallel to, forexample, the crystal orientation of GaAs of the compound semiconductorsubstrate 12.

GaAs as a major component of the base layer 20 may be n-typesemiconductor or p-type semiconductor. In this embodiment, since thecollector layer 16 is formed of n-type semiconductor, GaAs of the baselayer 20 is p-type semiconductor.

The shape of the base layer 20 in a plan view is, for example, arectangular shape long in one direction (see FIG. 1). The long-sidedirection of the rectangular shape (base layer 20) extends along crystalorientation [01-1] of the base layer 20. The short-side direction of thebase layer 20 extends along crystal orientation [011] of the base layer20. The shape of the base layer 20 when viewed from crystal orientation[011] is an inverted mesa shape (see FIG. 2) and the shape thereof whenviewed from crystal orientation [01-1] perpendicular to crystalorientation [011] is a forward mesa shape (see FIG. 5). In other words,the cross-section perpendicular to the short-side direction of the baselayer 20 has an inverted mesa shape and the cross-section perpendicularto the long-side direction of the base layer 20 is a forward mesa shape.In other words, both side surfaces in the long-side direction of thebase layer 20 are inverted mesa surfaces and both side surfaces in theshort-side direction of the base layer 20 are forward mesa surfaces. Thethickness of the base layer 20 is, for example, equal to or greater than0.05 μm and equal to or less than 0.15 μm.

The base electrode 22 is formed on the base layer 20 (see FIGS. 2 and5). The shape of the base electrode 22 in a plan view is, for example, arecess shape (see FIG. 1). The recessed portion of the recess shape isopened (recessed) when viewed from crystal orientation [01-1] of thebase layer 20. The recessed portion surrounds a part of the emitterlayer 24 and the like. The material of the base electrode 22 is notparticularly limited and examples thereof include Ti/Pt, WSi, Pt/Ti/Au,and AuGe/Ni/Au. Preferably, Pt/Ti/Au can be used as the material of thebase electrode 22.

The base electrode 22 is connected to the base line 22B via a contacthole 22A.

The contact hole 22A is formed of, for example, the same material as thebase line 22B. The lateral outer circumference of the contact hole 22Ais surrounded with, for example, a protective film as an insulator notillustrated.

The shape of the base line 22B in a plan view is, for example, anL-shape (see FIG. 1). An end of the L-shape (base line 22B) is connectedto the base electrode 22. The base line 22B is drawn out from aconnection area thereof to the base electrode 22 and from an end in theshort-side direction of the base layer 20 and the collector layer 16 tothe outside of the sub collector layer 14 (see FIGS. 1 and 5). Morespecifically, in this embodiment, the base line 22B is straightly drawnout along (in parallel to) the short-side direction of the collectorlayer 16, that is, crystal orientation [011] of the collector layer 16.As a result, the base line 22B goes over the side surfaces which are theforward mesa surfaces of the base layer 20, the collector layer 16, andthe sub collector layer 14.

The base line 22B is, for example, bent at right angle and extends alongthe long-side direction of the collector layer 16 after being drawn outto the outside of the sub collector layer 14. The other end of the baseline 22B as an extending destination is connected to a metal pad or thelike via a collective base line not illustrated.

The emitter layer 24 is formed on the base layer 20 (see FIG. 2). Theemitter layer 24 has, for example, a rectangular shape long in onedirection (see FIG. 1). The long-side direction of the emitter layer 24extends along crystal orientation [01-1] of the base layer 20.

The material of the emitter layer 24 is not particularly limited as longas it is semiconductor. In this embodiment, since the emitter layer 24is hetero-joined to the base layer 20, it is preferable that the emitterlayer 24 be formed of semiconductor having a material lattice-matchedwith the major component of the base layer 20 as a major component.Specifically, when the base layer 20 contains Al_(y)Ga_(1-y)As or GaAsas a major component, it is preferable that the emitter layer be formedof semiconductor containing InGaP or Al_(y)Ga_(1-y)As as a majorcomponent.

The emitter electrode 26 is formed on the emitter layer 24 (see FIG. 2).The material of the emitter electrode 26 is not particularly limited andexamples thereof include Ti/Pt, WSi, and AuGe/Ni/Au.

The emitter electrode 26 is connected to the emitter line 26B via thecontact hole 26A. The emitter line 26B is connected to a collectiveemitter line 28B of an upper layer via a contact hole 28A in an upperlayer thereon.

The contact hole 26A is formed of, for example, the same material as theemitter line 26B. The lateral outer circumference of the contact hole26A is surrounded with, for example, a protective film as an insulatornot illustrated.

The shape of the emitter line 26B in a plan view is, for example, arectangular shape (see FIG. 1). The long-side direction of the emitterline 26B extends along crystal orientation [01-1] of the base layer 20.

The contact hole 28A is formed of, for example, the same material as thecollective emitter line 28B. The lateral outer circumference of thecontact hole 28A is surrounded with, for example, a protective film asan insulator not illustrated.

The shape of the collective emitter line 28B in a plan view is, forexample, a rectangular shape (see FIG. 1). The long-side direction ofthe collective emitter line 28B extends along crystal orientation [011]of the base layer 20.

Manufacturing Method

A method of manufacturing the HBT 10 according to the first embodimentwill be schematically described below.

The sub collector layer 14, the collector layer 16, the base layer 20,and the emitter layer 24 are sequentially formed on the compoundsemiconductor substrate 12, for example, using a metal organic chemicalvapor deposition (MOCVD) method (see FIG. 2).

Then, the emitter electrode 26 is deposited on the emitter layer 24using a photoresist process.

Then, the emitter layer 24 not covered with the emitter electrode 26 isetched until the base layer 20 is exposed. Then, the base electrode 22is deposited on the exposed base layer 20.

Then, the base layer 20 and the collector layer 16 under the base layer20 are wet-etched until the sub collector layer 14 is exposed. Then, thesub collector layer 14 is wet-etched. The short-side directions and thelong-side directions are determined so that the short-side directions ofthe base layer 20 and the collector layer 16 extend along crystalorientation

(both are parallel to the same direction) at the time of etching thebase layer 20 and the collector layer 16, and the wet etching isperformed thereon. Since this etching is wet etching, etching anisotropy(with different etching rates) occurs depending on the crystalorientations of the base layer 20, the collector layer 16, or the subcollector layer 14. As a result, the base layer 20, the collector layer16, and the sub collector layer 14 have an inverted mesa shape whenviewed from crystal orientation [011] and have a forward mesa shape whenviewed from the direction perpendicular to crystal orientation [011].

Then, a pair of collector electrodes 18 is formed at both ends in thewidth direction of the sub collector layer 14 with the collector layer16 interposed therebetween on the sub collector layer 14 using alift-off method.

Then, a protective film such as an SiN film is formed from a side of thecompound semiconductor substrate 12 on which various layers such as thesub collector layer 14 are formed using a plasma CVD method or the like.

Then, the formed protective film is dry-etched so as to expose at leasta part of each of the collector electrode 18, the base electrode 22, andthe emitter electrode 26.

Then, the corresponding collector line 18B, the corresponding base line22B, and the corresponding emitter line 26B are formed on the exposedcollector electrode 18, the exposed base electrode 22, and the exposedemitter electrode 26 via the contact holes 18A, 22A, and 26A using aphysical vapor deposition method such as a sputtering method or adeposition method and a lift-off method. Here, the base line 22B isformed to be drawn out from an end in the short-side direction of thecollector layer 16.

Then, a protective film such as an SiN film is formed from the side ofthe compound semiconductor substrate 12 on which various layers such asthe sub collector layer 14 are formed using an MOCVD method or the like.

Then, the formed protective film is dry-etched so as to expose at leasta part of each of the collector line 18B, the base line 22B, and theemitter line 26B.

Then, a polyimide film is applied thereto for planarization. Thepolyimide film on the portion from which the protective film is etchedis etched so as to expose at least a part of each of the collector line18B, the base line 22B, and the emitter line 26B.

Then, the corresponding collective collector line 19B, the correspondingcollective base line (not illustrated), and the corresponding collectiveemitter line 28B are formed on the exposed collector line 18B, theexposed base line 22B, and the exposed emitter line 26B via the contactholes 19A and 28A and the like using a physical vapor deposition methodsuch as a sputtering method or a deposition method and a lift-offmethod. The HBT 10 illustrated in FIGS. 1 to 5 is manufactured throughthe use of the above-mentioned manufacturing steps.

Operation

The above-mentioned HBT 10 according to this embodiment includes thecollector layer 16 in which the short-side direction extends alongcrystal orientation [011] of the collector layer 16, in which thecross-section perpendicular to the short-side direction has an invertedmesa shape, and in which the cross-section perpendicular to thelong-side direction has a forward mesa shape and the base line 22B thatis drawn out from an end in the short-side direction of the collectorlayer 16 to the outside of the collector layer 16.

As a result, the base line 22B goes over the side surface which is theforward mesa surface of the collector layer 16. Accordingly, even whenthe short-side direction of the collector layer 16 extends along crystalorientation [011], it is possible to suppress disconnection of the baseline 22B in comparison with a case where the base line goes over theside surface which is the inverted mesa surface of the collector layer16.

In the HBT 10 according to this embodiment, similarly to the base layer20, the short-side direction extends along crystal orientation [011],the cross-section perpendicular to the short-side direction has aninverted mesa shape, and the cross-section perpendicular to thelong-side direction has a forward mesa shape.

As a result, the base line 22B goes over the side surface which is theforward mesa surface of the base layer 20. Accordingly, even when theshort-side direction of the base layer 20 extends along crystalorientation [011], it is possible to suppress disconnection of the baseline 22B in comparison with a case where the base line goes over theside surface which is the inverted mesa surface of the base layer 20.

The HBT 10 according to this embodiment further includes the subcollector layer 14 that is formed between the compound semiconductorsubstrate 12 and the collector layer 16 and that has the same crystalorientations as the collector layer 16, in which the cross-sectionperpendicular to the short-side direction of the collector layer 16 hasan inverted mesa shape and the cross-section perpendicular to thelong-side direction of the collector layer 16 has a forward mesa shape.

As a result, the base line 22B goes over the side surface which is theforward mesa surface of the sub collector layer 14. Accordingly, incomparison with a case where the base line goes over the side surfacewhich is the inverted mesa surface of the sub collector layer 14, it ispossible to suppress disconnection of the base line 22B.

In the HBT 10 according to this embodiment, the compound semiconductorsubstrate 12 includes a GaAs substrate. When the GaAs substrate isselected as the compound semiconductor substrate 12, the cost is lowerthan that of InP or the like and an increase in diameter is facilitated.

Second Embodiment

An HBT according to a second embodiment of the present invention will bedescribed below.

The HBT according to the second embodiment is different from the HBTaccording to the first embodiment, in a base line drawing method. Theother configurations are the same as in the first embodiment.

FIG. 6 is a plan view of an HBT 40 as an example of a bipolar transistoraccording to the second embodiment of the present invention.

As illustrated in FIG. 6, the HBT 40 includes a base line 42 connectedto a base electrode 22 via a contact hole 22A. The base line 42 is drawnout from an end in the short-side direction of the collector layer. As aresult, the base line 42 goes over the side surfaces which are theforward mesa surfaces of the base layer 20, the collector layer 16, andthe sub collector layer 14. More specifically, the base line 42 includesa first portion 42A, a second portion 42B, and a third portion 42C.

The first portion 42A is a portion that is present on the base layer 20and that is connected to the base electrode 22. The first portion 42Ahas a substantially rectangular shape. The long-side direction of thefirst portion 42A extends along crystal orientation [011] of thecollector layer 16. An end of the first portion 42A is connected to anend of the second portion 42B.

The second portion 42B is a portion of the base line 42 that is drawnout from the base layer 20 to the outside of the sub collector layer 14.The second portion 42B is straightly drawn out from an end in theshort-side direction of the collector layer 16 and is drawn out obliqueabout the end. The other end of the second portion 42B as the drawingdestination is connected to one end of the third portion 42C.

The third portion 42C is present on the compound semiconductor substrate12 and has a substantially rectangular shape. The long-side direction ofthe third portion 42C extends along crystal orientation [01-1] of thecollector layer 16. The other end of the third portion 42C is connectedto a collective base line not illustrated. The corner in the vicinity ofthe boundary between the second portion 42B and the third portion 42C ispreferably chamfered from the viewpoint of suppression of peeling-off ofthe base line 42.

In the HBT 40 according to the second embodiment, the second portion 42Bof the base line 42 in a plan view is drawn out oblique about the end inthe short-side direction of the collector layer 16. Accordingly, evenwhen the thickness of the second portion 42B in the vicinity of theforward mesa surface of the collector layer 16 or the like is smallerthan those of the other portions at the time of manufacturing, it ispossible to increase the width of the second portion 42B in the vicinityof the forward mesa surface in comparison with a case where the baseline is drawn out in parallel to the short-side direction of thecollector layer 16. Accordingly, in the base line 42, thecross-sectional area in the vicinity of the forward mesa surface of thecollector layer 16 or the like can be approximated to thecross-sectional areas of the other portions and it is thus possible tofurther suppress disconnection of the base line 42.

Third Embodiment

A semiconductor device according to a third embodiment of the presentinvention will be described below.

FIG. 7 is a plan view of a semiconductor device 50 according to thethird embodiment of the present invention.

The semiconductor device 50 according to the third embodiment includesat least one HBT 10 which has been described in the first embodiment andat least one HBT 60 different from the HBT 10 in the base line drawingmethod.

The HBT 60 includes a sub collector layer 62, a collector layer 64, acollector electrode 66, a base layer 68, a base electrode 70, an emitterlayer 72, and an emitter electrode 74. The HBT 60 further includes acollector line 76, an emitter line 78, and a base line 80.

The collector layer 64 and the base layer 68 contain, for example, GaAsas a major component. The collector layer 64 and the base layer 68 have,for example, a rectangular shape. In the collector layer 64 and the baselayer 68, the long-side direction thereof extends along crystalorientation [011], the cross-section perpendicular to the short-sidedirection thereof has an inverted mesa shape, and the cross-sectionperpendicular to the long-side direction has a forward mesa shape.Similarly, in the sub collector layer 62, the cross-sectionperpendicular to the short-side direction of the collector layer 64 hasan inverted mesa shape, and the cross-section perpendicular to thelong-side direction of the collector layer 64 has a forward mesa shape.

The shape of the base line 80 in a plan view is, for example,substantially a T-shape. The straight portion of the base line 80 isconnected to the base electrode 70 via a contact hole 80A.

The base line 80 is drawn out from a connection area thereof to the baseelectrode 70 and from the ends in the long-side direction of the baselayer 68 and the collector layer 64 to the outside of the sub collectorlayer 62. More specifically, in this embodiment, the base line 80 isstraightly drawn out along (in parallel to) the long-side direction ofthe collector layer 64, that is, crystal orientation [011] of thecollector layer 64. As a result, the base line 80 goes over the sidesurfaces which are the forward mesa surfaces of the base layer 68, thecollector layer 64, and the sub collector layer 62.

The base line 80 extends along the long-side direction of the collectorlayer 64 after being drawn out to the outside of the sub collector layer62. The other end of the base line 80 as an extending destination isconnected to a metal pad or the like via a collective base line notillustrated.

The other configurations of the HBT 60 are the same as described in thefirst embodiment and description thereof will not be repeated.

In the above-mentioned semiconductor device 50 according to the thirdembodiment, in comparison with a case where the HBTs 60 are simplyarranged, the number of directions in which the base line 80 is drawnout and it is thus possible to increase a degree of freedom in layout ofthe semiconductor device 50. Accordingly, for example, it is possible todecrease the size of the semiconductor device 50.

Fourth Embodiment

A semiconductor device according to a fourth embodiment of the presentinvention will be described below.

FIG. 8 is a plan view of a semiconductor device 90 according to thefourth embodiment of the present invention.

The semiconductor device 90 according to the fourth embodiment includesat least two HBTs 100 which are the same as the HBT 10 described in thefirst embodiment.

In the HBTs 100, a sub collector layer 102, a collector layer 104, and abase layer 106 are used in common. Each HBT 100 individually includes anemitter layer 108 and an emitter electrode 110. Each HBT 100 alsoindividually includes a collector electrode 112A. A collector electrode112B opposed to the collector electrode 112A is used in common to theHBTs 100. The collector electrodes 112A and the collector electrode 112Bare connected to a collector line 114. The emitter electrodes 110 areconnected to an emitter line 116.

The collector layer 104 and the base layer 106 contain, for example,GaAs as a major component. The collector layer 104 and the base layer106 have, for example, a rectangular shape. In the collector layer 104and the base layer 106, the short-side direction thereof extends alongcrystal orientation [011], the cross-section perpendicular to theshort-side direction thereof has an inverted mesa shape, and thecross-section perpendicular to the long-side direction has a forwardmesa shape. Similarly, in the sub collector layer 102, the cross-sectionperpendicular to the short-side direction of the collector layer 104 hasan inverted mesa shape, and the cross-section perpendicular to thelong-side direction of the collector layer 104 has a forward mesa shape.

In the HBTs 100, the base electrode 118 and the base line 120 are usedin common.

The base line 120 is drawn out from a connection area thereof to thebase electrode 118 and from the ends in the short-side direction of thebase layer 106 and the collector layer 104 to the outside of the subcollector layer 102. As a result, the base line 120 goes over the sidesurfaces which are the forward mesa surfaces of the base layer 106, thecollector layer 104, and the sub collector layer 102.

The base line 120 extends along the short-side direction of thecollector layer 104 after being drawn out to the outside of the subcollector layer 102. The other end of the base line 120 as an extendingdestination is connected to a metal pad or the like via a collectivebase line not illustrated.

In the above-mentioned semiconductor device 90 according to the fourthembodiment, since the base electrode 118 and the base line 120 are usedin common to the HBTs 100, it is possible to suppress an increase in acontact area between the base layer 106 and the collector layer 104, tosuppress an increase in a base-collector capacitance, and to carry out alayout in consideration of balance of power supply positions.Accordingly, for example, it is possible to decrease the size of thesemiconductor device 90.

Fifth Embodiment

A semiconductor device according to a fifth embodiment of the presentinvention will be described below.

FIG. 9 is a plan view of a semiconductor device 121 according to thefifth embodiment of the present invention.

As illustrated in FIG. 9, the semiconductor device 121 is an RF poweramplifier module. The semiconductor device 121 includes at least one HBT10 described in the first embodiment and uses the HBT as a poweramplifier.

The semiconductor device 121 includes a matching circuit area 122 on theinput side of the HBT 10, a matching circuit area 124 on the output sideof the HBT 10, and a control chip 126 in addition to the HBT 10.

In the above-mentioned semiconductor device 121 according to the fifthembodiment, since the HBT 10 is used as a power amplifier, it ispossible to decrease the size of an RF power amplifier module.

Modification Example

The first to fifth embodiments are presented to facilitate understandingof the present invention, but are not presented to limit the presentinvention. The present invention can be modified/improved withoutdeparting from the gist thereof, and equivalents thereof are alsoincluded in the present invention.

For example, use of the sub collector layer 14 may be skipped.Similarly, use of the emitter electrode 26 may be skipped.

When a semiconductor substrate containing Si as a major componentinstead of the compound semiconductor substrate 12 is used as asubstrate, one or more buffer layers may be inserted between thesubstrate and the sub collector layer 14.

In the structure illustrated in FIG. 5, a source/drain ohm contact layer132 and an FET-section channel layer 134 may be added between thecompound semiconductor substrate 12 and the sub collector layer 14 toconstitute a BiFET 130 as illustrated in FIG. 10.

It has been described above that the second portion 42B of the base line42 is straightly drawn out oblique about the end in the short-sidedirection of the collector layer 16, but the second portion 42B may havea trapezoidal shape, the upper base of which is connected to the firstportion 42A and the lower base is connected to the third portion 42C. Inthis case, the width of the base line 42 increases toward the drawingdestination.

The collector electrode 18 does not need to be located on both sides,but the collector electrode 18 may be located on only one side.

What is claimed is:
 1. A bipolar transistor comprising: a collectorlayer that has a long-side direction and a short-side direction in aplan view, wherein the short-side direction is parallel to crystalorientation [011], a cross-section perpendicular to the short-sidedirection has an inverted mesa shape, and a cross-section perpendicularto the long-side direction has a forward mesa shape; a base layer thatis formed directly on the collector layer such that the base layercontacts the collector layer; a base electrode that is formed directlyon the base layer such that the base electrode contacts the base layer;and a base line that is connected to the base electrode and that isdrawn out from an end in the short-side direction of the collector layerto the outside of the collector layer in a plan view.
 2. The bipolartransistor according to claim 1, wherein the base layer has a long-sidedirection and a short-side direction in a plan view and the base layerhas the same crystal orientation as the collector layer has, theshort-side direction of the base layer is parallel to crystalorientation [011], a cross-section perpendicular to the short-sidedirection of the base layer has an inverted mesa shape, and across-section perpendicular to the long-side direction of the base layerhas a forward mesa shape.
 3. The bipolar transistor according to claim 1further comprising a sub collector layer that is formed between asubstrate and the collector layer, the crystal orientation of the subcollector layer being the same as the crystal orientation of thecollector layer, wherein a cross-section perpendicular to the short-sidedirection of the collector layer has an inverted mesa shape, and across-section perpendicular to the long-side direction of the collectorlayer has a forward mesa shape.
 4. The bipolar transistor according toclaim 3, wherein the substrate is a GaAs substrate.
 5. The bipolartransistor according to claim 1, wherein the shape of the base electrodeis a recess shape in a plan view and the recessed portion surrounds apart of an emitter layer formed on the base layer.
 6. The bipolartransistor according to claim 5, wherein the recessed portion of therecess shape is opened when viewed from crystal orientation [01-1] ofthe base layer.